1. Field of the Invention
The present invention relates to the metal-catalyzed radical and living radical polymerization of chlorine-containing monomers such as vinyl chloride and vinyl halides. In particular, this invention relates to a process for the synthesis, in the presence of a catalyst, of poly(vinylchloride) (PVC) with controlled molecular weight and narrow molecular weight distribution. The polymerization can be initiated from various mono, di, and tri and polyfunctional, activated halide-containing initiators such as α,α-dihaloalkanes, α,α,α-trihaloalkanes, perhaloalkanes, perfloroalkyl halides, benzyl halides, allyl halides, sulfonyl halides, α-haloesters, α-halonitriles, α-haloketones, imidyl halides, or combinations thereof, in the presence of metal-based catalysts.
2. Description of the Prior Art
Heretofore, it was known to polymerize vinyl chloride (VC) and other vinyl halide monomers using conventional free radical processes. In a free radical polymerization the molecular weight is independent of conversion and the final molecular weight is already observed at conversions of about 5%. In this type of chain polymerization, in addition to propagation, the growing chain is involved in several other reactions such as chain transfer to monomer, solvent or initiator as well as bimolecular termination. As a consequence, even in the presence of certain additives for molecular weight control, the concentration of the growing species is not constant throughout the polymerization and the polydispersity is usually 2 or more, while the molecular weight distribution has a Gaussian distribution. In addition, PVC synthesized by free radical polymerization contains allyl and tertiary chlorines that are responsible for the low thermal stability of commercial PVC which requires thermal stabilizers for its practical use.
Heretofore, it has not been known to prepare poly(vinyl chloride) by a metal-catalyzed living radical process initiated from an active halide compound in which the molecular weight and the molecular weight distribution of PVC could be controlled. Living polymerizations are chain polymerization reactions in which the concentration of the growing species is approximately constant throughout the polymerization process. This is a consequence of the absence or low extent of transfer or termination reactions. A living polymerization is demonstrated by a linear increase of the polymer molecular weight with conversion. In addition, if the rate of initiation is larger than the rate of propagation, in a living polymerization the polydispersity decreases with conversion and approaches Mw/Mn=1+1/DP where DP=degree of polymerization (i.e. Poisson distribution). However, if the rate of initiation is slower than the rate of propagation the molecular weight distribution is Mw/Mn=2.
Previous attempts at metal-catalyzed radical polymerizations of vinyl chloride have resulted only in low molecular weight PVC oligomers called telomers. This low molecular weight poly(vinyl chloride) is allegedly formed by a free radical telomerization reaction by reacting the unsaturated vinyl monomer (vinyl chloride, taxogen) in the presence of an initiator with an AB molecule (telogen). However, such telomerization reactions generated only low molecular weight oligomers A—(Mn)—B where n<10 and the chain ends were derived from the telogen. Additionally, in these free radical telomerizations of vinyl chloride, the growing chains were irreversibly terminated by chain transfer to the telogen at low degrees of polymerization.
The metal catalyzed free radical telomerization of vinyl chloride (VC) initiated by redox systems based on certain haloalkanes and metals or metal complexes leads predominantly to low molecular weight species (DP=1 to 4 where DP=degree of polymerization) containing chain ends derived from the telogen. Such telomerizations were previously performed (see for example Freidlina, R. Kh.; Chukovskaya, E. C. Synthesis 1974, 477 and Freidlina, R. Kh.; Chukovskaya, E. C. Synthesis 1977, 145) using CCl4 as alkyl halide telogen and FeCl2, FeCl3, Fe(CO)5 (DP=1-3 and 10 to 30), CuCl, CuCl2 (DP=1-3), CrCl3, Fe(0), Cu(0) (DP<10) Ni(0), Mg(0), and Zn(0) as catalysts in the presence of alcohols or amines. Fe(CO)5 was also used as catalyst for other telogens such as FSO2Cl (DP=1-3), CHBr3 (DP=1-2), PhCH2Cl (DP=1-4), CH2Br2 (DP=1-2), ethyltrichloroacetate, methyldibromoacetate and dichloroacetoacetate, diethyldichloromalonate, CHl3, (DP=1-2), as well as polyhaloalkanes such as: CCl3—CH3, CCl3—CHCl2, CCl3—CH2—CH2Cl and CCl3—(CH2)3—CH2Cl (DP=1-2). Metal carbonyls such as Cr(CO)6, Mn2(CO)10, Mo(CO)6 and W(CO)6 were also used as catalysts in the presence of CBr4 as telogen (DP=1-2). CCl4 was also used as telogen/chain transfer agent in VC polymerizations mediated by AlEt3, AlEt3/chloranil, AlEt3/CuCl, Et2AlCl/Ti(OBu)4, various Al alkyls and trialkylboron. Arylmethyl chlorides and bromides were also tested as initiators in the presence of Ag(0) or Hg(0).
Fractionation of telomers synthesized using CCl4 and FeCl2 has revealed that telomers of the structure CCl3(CH2—CHCl)nCl were mobile liquids for n=1 to 3, viscous liquids for n=4 to 7 and solids for n>8. Only few other viscosimetric and gel permeation chromatography (GPC) data are available in the literature for fractionated telomers with DP=3-16 synthesized in the presence of a metal catalyst.
The radical polymerization of vinyl chloride is characterized by one of the largest values of the chain transfer constant to monomer (CM) from all conventional monomers. At 60° C., the value CM for VC (CVC=1.08-1.6×10−3) is three to two orders of magnitude larger than for methyl methacrylate (MMA) (CMMA=7×10−6−2.5×10−5) and two orders of magnitude larger than for styrene (St) (CSt=3−6×10−5). Therefore, due to the chain transfer to monomer process, the maximum molecular weight that can be obtained in the free radical polymerization of vinyl chloride is effectively limited and controlled by the reaction temperature (i.e. molecular weight decreases with increasing the reaction temperature). In the presence of additional chain transfer agents such as telogens or any other chain transfer agents, the molecular weight will be even lower than in the free radical polymerization. For example, the theoretical maximum molecular weight that can be obtained by free radical polymerization of VC at 90° C. is approximately 17,000, while at 130° C. the maximum molecular weight is 6,000. In addition, the values of the rate constants of propagation (kp) and termination (kt) for VC are two orders of magnitude larger than for MMA and St [kp,VC=1.1×104 L·mol−1·s−1 (50° C.), kp,MMA=5.15×102 L·mol31 1·s−1 (60° C.), kp,st=1.65×102 L·mol−1·s−1 (60° C.) and kt,VC=2.1×109 L·mol−1·s−1 (50° C.), kt,MMA=2.55×107 L·mol−1·s−1 (60° C.), kt,St=6×107 L·mol−1·s−1 (60° C.)].
In the free radical telomerizations of VC with various polyhalide telogens the values of the chain transfer constant range from 0.28-0.29 (CCl4 and CHCl3) to 0.74-1.77 (CHBr3), and from 40 (CCl3Br) and 4 to 74 (CBr4), and to 7 to 54 (CCl2Br2). For the redox catalyzed telomerization of VC in acetonitrile at 100° C. using CCl4 as telogen and FeCl3 as catalyst, it was shown that CCCl4=0.025, CM=24×10−4 and CFeCl3=38. In addition, it was also demonstrated that at 60° C., propagating vinyl chloride radicals are much more reactive (k=1.04×106 M−1·s−1) than MMA (k=3.05×103 M−1·s−1) or styrene (k=5.4×104 M−1·s−1) towards FeCl3 (CFeCl3,VC=85.5) which can quantitatively terminate the PVC growing chain. While the values of CCuCl2 are not available for VC, for MMA CFeCl3,MMA=4.16 and CCuCl2,MMA=1050. The values of the chain transfer constants to a chain transfer agent are monomer specific but the relative trends are maintained from monomer to monomer. It is therefore reasonable to assume that CCuCl2,VC>CFeCl3,VC and that CCuCl2,VC>Ctelogen and CCuCl2,VC>CVC. In addition, as the values of the chain transfer constants to the metal species were determined in the absence of solubilizing ligands, it is expected that even larger values are observed in the presence of coordinating ligands.
The polyhaloalkane telogens employed in the metal catalyzed telomerization of VC are therefore characterized by a value of their chain transfer constant that is lower than that of the catalytic metal species. In free radical telomerization of VC, the growing chains are irreversibly terminated by chain transfer to the telogen at low degrees of polymerization. By contrast, in metal catalyzed polymerization of VC initiated in a redox process from metal species and an alkyl halide (R—X where R contains an activating electron withdrawing group such as cyano, esters, perfloroalkyl or another unit capable of stabilizing a radical such as benzyl or allyl and X=halide), the metal halide species has a high value of the chain transfer constant. Consequently, the polymeric chain ends are derived from the R fragment and a halide ligand from the metal salt.
Conventional free radical polymerization of vinyl chloride (VC) is accompanied by the formation of thermally labile tertiary and allylic chlorine defects (Schemes 1 and 2) which are responsible for the low thermal stability of poly(vinyl chloride) (PVC) which provides its most relevant technological limitations. 
These structural defects are generated during the conventional radical polymerization of VC and are responsible for the initiation of a zipper mechanism of thermal degradation of PVC. Detailed investigations on the mechanism of formation of these defects and subsequent degradation of PVC are available.
Both allylic chloride and tBuCl PVC defects should act as initiators for metal catalyzed living radical polymerization (see for example Percec, V. et al J. Polym. Sci.: Part A.: Polym. Chem. 2001, 39, 1120). Therefore, in a metal catalyzed radical polymerization of VC, the labile chlorines generated during the radical propagation process would be involved as new initiating sites for the polymerization of VC and the resulting PVC may have a branched structure but would not contain labile chlorines. Theoretically, if the initiation process is much faster than the propagation and no such side reactions occur, the molecular weight of the polymer increases linearly with conversion while the polydispersity decreases with conversion to values between 2 and 1. In general, the bimolecular radical side reactions are suppressed by reversibly endcapping the growing polymeric chain with a low molecular weight species (i.e. a persistent radical). General reviews of the field include: Otsu, T. J. Polym. Sci.: Part A.: Polym. Chem. 2000, 38, 2121; Darling, et al J. Polym. Sci.: Part A: Polym. Chem. 2000, 38, 1706; Gaynor, S. G.; Sawamoto, M.; Kamigata, M. Chemtech 1999, 29, 30; Matyjaszewski, K. ACS Symp. Ser. 2000, 768, 347; Fischer, H. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 1885.
Previous attempts at the living radical polymerization of vinyl halides did not involve metal catalysis and were based on degenerative chain transfer processes. In addition, the polydispersity never decreased to values below 1.7. See for example Bak, P. I. et al U.S. Pat. No. 5,455,319.